Superheterodyne radio altimeter or locator



April 5, 1949.

l w. L. CARLSON SUPERHETERODYNE 'RADIO lALTIIIMETER ORLOGATOR 2 sheets-sheet 1 Filed Oct. 20. 19425 .ob um. NN

tu? sbg* :inventorI Gttorueg WENDELL LEAnLsuN.

v l .Qu :EN

April 5, 1'949.

Enea oct. 2o, 194s.

W. L. CARLSON SUPERHETERODYNE RADIO ALTIMETER OR LOCATOR 2 Sheets-Sheet 2 nventor WENDELL LEARLSUN attorney Patented Apr. 5, 1949 SUPERHETERODYNE RADIO ALTIIVIETER R LOCATOR Wendell L. Carlson, Princeton, N. J., assignor to Radio Corporation of America, a corporation o! Delaware Application 0ctober 20, 1943, Serial No. 507,221

13 claims. (ci. 343-14) My invention relates to radio systems of the type utilizing reflected radio waves for distance measuring, detection of reflecting objects, or the like. It relates particularly to altimeters or other distance measuring systems which are cyclically frequency modulated and to systems that utilize the Doppler effect.

In systems of the above-mentioned types it has been the usual practice to apply the reflected signal to a mixer or detector where it is mixed with signal supplied directly from the transmitter to obtain an audio signal which is then amplified by an audio amplifier. It has been found that the amount of audio amplification vthat can be employed is severely limited because of microphonics in the mixer and audio amplifier. Also, it has been found difficult to obtain sufficiently exact balance of the mixer to prevent unavoidable audio amplitude modulation from the transmitter signal from rectifying and passing on to the audio amplifier.

The difliculties caused by microphonlcs and by lack of exact balance can be eliminated or reduced by employing the superheterodyne principle and obtaining the desired amplification in an intermediate-frequency or I.F. amplifier where the signal being amplified has a frequency above that' of the microphonlcs. superheaterodyne pirinciple in a conventional manner for this purpose would not be entirely satisfactory both because of oscillator frequency drift and, in the case of the frequency-modulated FM altimeter, also because of the fact that the L-F. pass' band would have to be wide enough to include the frequency deviation of the carrier wave.

An object of the present invention is to provide an improved method of and means for obtaining a large amount of amplification in FM altimeter or radar systems and in systems utilizing the Doppler efiect while, at the same time, avoiding difliculties due to microphonlcs.

A further object of the invention is to provide an improved radio distance measuring or object locating system of the type utilizing reflected radio waves in which the received signals are amplified in an intermediate frequency amplifier.

A further object of the invention is to provide an improved frequency modulated altimeter or object locator of the superheterodyne type wherein the width of the I.F. pass band is independent of the carrier wave frequency deviation introduced by the frequency modulation.

A further object of the invention is to provide an improved frequency modulated altimeter or The use of the' t object locator of the superheterodyne type wherein the I.F. signal is not shifted in frequency by frequency drift of the heterodyning oscillator.

A further object of the invention is to provide an improved radio locator system utilizing a balanced detector wherein any unbalance of its parallel input circuit or of its push-pull input circuit will cause a minimum reaction of one upon the other.

In practicing one embodiment of the invention, means is provided for supplying to the mixer or detector of the receiver a signal that is derived from the transmitted signal but which differs therefrom in frequency by an amount equal to the desired intermediate-frequency carrier wave.

This derived signal may be obtained by supplying to a modulator both the received signal and signal from an oscillator operating at the desired intermediate frequency. The resulting frequencydifference or lower side band signal appearing in the modulator output is then applied to the mixer tube of the receiver where it beats with the signal direct from the transmitter. The resulting mixer tube output includes an I.F. carrier wave of the desired intermediate frequency. The I.-F. carrier wave is frequency modulated and has a frequency deviation corresponding to the elapsed time required'for the signal to travel to the reflecting object and return. An audio beat frequency component is then obtained from the I.F. signal by combining it with the modulator oscillator frequency corresponding to the I.F. frequency. This beat frequency signal is supplied to a suitable indicator such as a frequency counter or frequency indicator.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a block and circuit diagram of a radio altimeter or locator system embodying the invention, and Figures 2 and 3 are block diagrams of other embodiments of the invention. In the three figures; similar parts are indicated by similar reference characters.

Fig. 1 shows an embodiment of the invention applied to a frequency-modulated altimeter or radar system comprising a high frequency transmitter oscillator I 0 which is cyclically frequency modulated in the usual manner for supplying a.l

wave supplied by a wave generator I3. The outl put of the generator I3 may be a wave of any suitable wave form such as a triangular wave, a sawtooth wave or a sine wave. Merely by way of example it is assumed that the carrier wave frequency is 500 megacycles per second, thatI the frequency swing or deviation produced by modulator I2 is 2 megacycles, and that the modulator I2 is driven by a triangular wave recurring at a rate of 120 per second.

The system also comprises a balanced detector or mixer I6 which is supplied through a balanced modulator l1 with the signal picked up by an antenna I8 after it has been reflected from the earths surface or from an object or objects4 to be located. However, according to the embodiment of the invention shown in Fig. 1, side band signals of the reflected carrier wave signal are produced in the modulator I1 and one of them (the lower one ln the example illustrated) is supplied to the mixer I6, where it is mixed with carrier wave signal supplied directly from the transmitter I 0. In the example illustrated, the 500 mc. carrier wave direct from the transmitter is mixed with the 470 mc. lower side band of the signal from the modulator I1 whereby a 30 mc. intermediate-frequency signal is obtained.

In order to obtain this I.F. carrier wave, an oscillator 2| supplies to the modulator I1 a sine wave signal having the frequency that is desired for the intermediate frequency, this being 30 megacycles per second in the example assumed. Thus the output of the modulator I1 contains side bands of 530 megacycles and 470 megacycles, all being frequency deviated 2 megacycles. For example, the 470 megacycle side band is being swung from 469 megacycles per second to 471 megacycles per second and back to 469 megacycles per second at the rate of 120 times per second in the system illustrated. Only the lower side band signal of 470 megacycles with its 2 megacycle deviation is passed to the mixer I6 since the coupling transformer 22 is tuned to pass only this one side `band signal.

In the mixer I6, the direct 500 megacycle signal supplied over conductors 23 and the reflected 470 megacycle signal supplied through transformer 22 (both containing the 2 megacycle deviation) are mixed or heterodyned whereby a signal having their difference frequency appears in the mixer output, the dierence frequency being normally 30 megacycles. This 30 megacycle signal is frequency modulated at a rate of 120 cycles per second and has a frequencyv deviation corresponding to the elapsed time required for the signal to travel to the reflecting object and return. This modulation results from the frequency or phase difference of the two signals being heterodyned in the mixer I6; the amount of frequency or phase difference depending upon the propagationtime of the wave that travelled from the transmitter to the reflecting object and back to the receiver.

The 30 megacycle I.F. signal is amplified in an I.F. amplifler 24 which may be designed to have good selectivity since the I.F. signal is not affected by the 2 megacycle deviation frequency or by frequency drift of the oscillator I0. It may be noted that in some systems the frequency deviation is `made much more than 2 megacycles, in which case there is a still greater advantage in employing the present invention. In the example being described, the I.F. amplifier 24 may have a pass band only 300 kilocycles per second wide, this being suflcient to allow oscillator 2| to frequency drift one percent. The pass band of am- 4 pliiier 24 is determined in the usual way by tuning the I.F. transformers 28 and 21.

The output of the I.F. amplifier 24 is supplied to a detector 28 where it is mixed with a 30 megacycle carrier signal supplied from the oscillator 2| over conductors 26. The resulting detector output vcontains the desired audio beat or beats which are supplied to a suitable frequency indicator or counter 3I.

Any one or all of the units I6, I1 and 28 of Fig. 1 may be either balanced or unbalanced as preferred. The mixer I6 is shown as of the balanced type to eliminate any amplitude modulation that may be present, as described and claimed in applicationSerial No. 445,010, filed May 29, 1942, now Patent No. 2,419,046 which issued April 15, 1947 in the names of Royden C. Sanders, Jr. and Irving Wolff, and entitled Altimeter or distance indicator. It should be noted that any unbalance in the mixer I6 of the parallel input circuit (the circuit to which is applied the 500 mc. signal) with respect to the push-pull input circuit (the circuit to which is applied the 470 mc. signal) will have comparatively little effect since the two circuits are tuned to frequencies 30 mc. apart. This makes it less difllcult to balance out amplitude modulation. It will be understood that the tuning of the parallel and push-pull circuits is done by the input capacities of the mixer tubes and by the tuning of the transformer 22.

The detector 28 preferably is balanced if appreciable audio amplitude modulation is present on the signal from oscillator 2I supplied through conductors 29. Otherwise, the mixing signal from the oscillator 2l may -be supplied to an unbalanced detector. It will be noted that the signal from oscillator 2l always has the same frequency as that of the I.F. signal so that good detection is ensured. It is not essential that the detector 28 be of the type that mixes a carrier signal with the received signal. For example, the detector 28 may be of the well known diode type preceded by a frequency discriminator, there being no signal supplied to it from the oscillator 2 I. It may be noted, however, that the frequency shift of the I.F. signal is so small that this method of detection is comparativelyinemcient'.

Fig. 2 shows an embodiment of the invention in which the modulator I1 is located at the output circuit of the transmitter I0 instead of at the input circuit of the receiver as in Fig. 1. In this case one o f the side bands is transmitted to the reecting object, this being the lower side band in the example illustrated. Both side bands could be transmitted and one of them filtered out at the receiver after reflection. At the mixer I8 the reiiected side band signal mixes with the carrier wave signal being supplied over the conductors 23 whereby the I.F. signal is produced.

Fig. 3 shows an embodiment of the invention which is similar to that of Fig. 1 except that use is made of both side band signals appearing in the output circuit of the modulator I1. These two side bands, of 439 mc. and 441 mc. in the specific example assumed, beat inthe mixer I6 with the 440 mc. carrier signal to derive an I.F.

in accordance with the timel difference of the direct and reflected signals supplied to the mixer I6 whereby the desired beat frequency signals appear in the output of detector 28 Just as in the circuits previously described.

lA Doppler effect system utilizing the present inventionmay be the same as the systems described except that .the frequency modulator i! and the modulating wave generator Il are omitted. Such a system may be utilized for various purposes such as measuring the speed of an object or detecting or locating a moving object.

I claim as my invention:

1. In a radio system for locating a reflecting object, a receiver including a first detector', transmitter means for producing a radio carrier wave signal, means including an oscillator for producing side band signalsof said radio wave signal, means for supplying two of said signals to said detector with one of said signals substantially undelayed and with the other ot said signais delayed due to traveling to the reflecting object and back by an amount equal to twice the propagation time from the transmitter ,tothe reflecting object for obtaining an intermediatefrequency signal, and means for detecting or converting said intermediate-frequency signal to an audio frequency signal, said last-means comprising a non-linear or rectifying device and means for applying to said device both said intermediate-frequency signal and signal from said oscillator.

2. In a radio system for locating a reflecting object, a receiver including a first detector, transmitter means for :producing a radio carrier wave signal, means for cyclically frequency modulating said carrier wave signal, means including an oscillator for producing side band signals of said frequency-modulating signal, means for supplying two of said signals to said detector with one of said signals substantially undelayed and with the other of said signals delayed due to traveling to the reflecting object and back by an amount equal to twice lthe propagation time from the transmitter to the reflecting object for obtaining an intermediate-frequency signal, and means for detecting or converting said intermediate-frequency signal to an audio frequency signal, said last means comprising a non-linear or rectifying other of said signals delayed due to traveling to the reflecting object and back by an'amount `equal to twice the propagation time from the transmitter to the reflecting object for obtaining intermediate frequency signal having the same carrier frequency as that of the output of said oscillator. and means for obtaining a beat frelquency signal representativey of the distance to device and means for applying to said device both said intermediate-frequency signal and signal from said oscillator.

3. In a radio system for locating a reflecting object, a receiver including a first detector, transmitter means for producing. a radio carrier wave signal, means including an oscillator for producing a side band signal of said radio wave signal, means for supplying said carrier wave signal and said side band signal to said detector with one of said signals substantially undelayed and with the other of said signals delayed due to traveling to the reflecting object and back by an amount equal to twice the propagation time from the transmitter to the reflecting object for obtainingan intermediate-frequency signal having the same carrier frequency as that of the output of said oscillator. and means for detecting or converting said intermediate-frequency signal to an audio frequency signal, said lastl means comprising a non-linear or rectifying device and means for applying to said device both said intermediate-frequency signal and lsignal from said oscillator. y

4. `In a radio system for locating a reflecting object, a receiver including a first detector, transmitter means for producing a radio carrier wave signal which is cyclically frequency modulated, means including an oscillator for producing a side band signal of said radio wave signal, means for supplying said carrier wave signal and said side band signal to said detector with one of said signals substantially undelayed and with the the reflecting object, said last means comprising a non-linear or rectifying device and means for applying to said device both said intermediatefrequency signal and signal from said oscillator.

5. A radio locating system comprising means for producing a radio carrier wave signal, means including an oscillator for producing a side band signal of said carrier wave signal, means for transmitting one of said signals to a reflecting object, means for receiving said transmitted signal after reflection from saidobject, said receiving means including a first detector, means for mixing said reflected signal and the other of said signals in said first detector whereby there is obtained an intermediate-frequency signal having the same carrier frequency as that of the output of said oscillator, and means for detecting or converting said intermediate-frequency signal to an audio frequency signal, said last means comprising a non-linear or rectifying device and means for applying to said device both said intermediate-frequency signal and signal from said oscillator.

6. A radio locating system comprising means for producing a radio carrier wave signal, means including an oscillator for producing a side band signal of said radio wave, means for transmitting one of said signals to a reflecting object, means for receiving said transmitted signal after reflection from said object, said receiving means including a first detector, means for mixing said carrier wave signal and said side band signal in said detector, one of said signals being applied to the first detector directly from the transmitting means and the other of said signals being the reflected signal, whereby there is obtained an inintermediate-frequency signal having the same carrier frequency as that of the output of said oscillator, and means ,for detecting or converting said intermediate-frequency signal to an audio frequency signal, said last means comprising a non-linear or rectifying device and means for applying to said device both said intermediatefrequency signal and signal from said oscillator.

7. A radio locating vsystem comprising means for transmitting a radio wave to a reflecting object whereby said wave is reflected, means for receiving said reflected wave, said receiving means including a first detector, means including an oscillator for producing a side band signal of said reflected radio wave. means for mixing said radio wave direct from the transmitter and said side band wave in said detector whereby there is obtained an intermediate-frequency signal having the same carrier frequency as that of said oscillator output, and means for detecting or converting said intermediate-frequency signal to an audio frequency signal, said last means comprising a non-linear or rectifying device and means for applying to said device both said intermediate-frequency .signal and signaly from said oscillator.

8. A radio locatingsystem comprising means for producing a cyclically frequency modulated radio wave, means for transmitting said wave to a reflecting object whereby said wave is reflected,

ananas means for receiving said reflected wave, said receiving means including a first detector, means including an oscillator for producing a side band signal of said reflected radio wave, the output of said oscillator having a substantially constant frequency, means for mixing said radio wave direct from the transmitter and said side band wave in said detector whereby there is obtained an intermediate-frequency signal having the same carrier frequency as that of said oscillator output, and means for detecting or converting said intermediate-frequency signal to an audio frequency signal, said last means comprising a non-linear or rectifying device and means for applying to said device both said intermediatefrequency signal and signal from said oscillator. 9. In a radio system for locating a reflecting object, a receiver including a first detector or mixer, transmitter means for producing a radio carrier wave signal, means including a modulator located between the transmitter output and the rst detector input and in the channel of the signal that is transmitted and reflected and also including an oscillator for producing side band signals of said radio wave, means for supplying said carrier wave signal and at least one of said side band signals to said detector with one of said signals substantially undelayed and with the other of said signals delayed due to traveling to the reflecting object and back by an amount equal to twice the propagation time from the transmitter to the reflecting object to obtain an intermediate-frequency signal having the same carrier frequency as that of the output of said oscillator and having a frequency or phase modulation thereon which is a function of said propa-l gation time, and means including a second detector for detecting said frequency or phase modulation on said intermediate-frequency signal.

10. A radio locating system comprising means for transmitting a radio wave to a reflecting object whereby said wave is reflected, means for receiving said reflected wave, said receiving means including a modulator and a first detector connected in the order named, means including said modulator and an oscillator for producing side band signals of said reflected wave, means for supplying said radio wave directly to said rst detector and for mixing it and at least one of'said side band signals in said detector whereby there is obtained an intermediate-frequency signal having the same carrier frequency as that of the output of said oscillator and having a frequency or phase modulation thereon which is a function of the distance to said reilecting object, and means including a second detector for detecting said frequency or phase modulation on said intermediate-frequency signal.

11. A radio locating' system comprising means for producing a radio carrier wave signal, a transmitting antenna, a modulator located between said antenna and said carrier wave producing means, means including said modulator and an oscillator for producing a side band signal of said radio wave, said side band signal being radiated to a reflecting object, means for receiving said radiated signal after reection from said object, said receiving means including a first detector, means for mixing said carrier wave signal and said reflected side band signal inv said detector, said carrier wave signal being applied to the first detector directly from the transmitting means whereby there is obtained an intermediate-frequency signal having the same carrier frequency as that of said oscillator output and having a frequency or phase modulation thereon which is a function of the distance to said reflecting object, and means including a second detector for detecting said frequency or phase modulation on said intermediate-frequency signal.

12. A radio locating system comprising means for receiving signal reflected from an object to be located, said receiving means including a rst detector, said system also comprising a transmitter oscillator for vproducing a radio carrier wave signal, a transmitting antenna, a modulator located in the transmitting and receiving path at some point between said transmitter oscillator and said rst detector, means including said modulator and a second oscillator for producing side band signals of said radio wave, at least one of the above-mentioned signals being radiated to said reflecting object, means for mixing said carrier wave signal and at least one of said side band signals in said detector, the side band signal or signals mixed in said detector being delayed by an amount equal to twice the propagation time from the transmitter to said refleeting object, said carrier wave signal being applied to the first detector directly from the transmitting means whereby there is obtained an intermediate-frequency signal having the same carrier frequency as that of said second oscillator output, and means including a second detector for converting said intermediate-frequency signal to an audio frequency signal.

13. In a radio system for locating a reflecting object,. a receiver including a rst detector or mixer, a, transmitter voscillator for producing a radio carrier wave signal, a transmitter antenna for radiating signal to said reflecting object. means including a modulator located between the transmitter oscillator and the rst detector input and in the channel of the signal that is transmitted and reflected and also including a second oscillator for producing side band signals of said radio Wave, means for supplying said carrier wave signal and at least one of said side band signals to said detector with one of said signals substantially undelayed and'with the other of said signals delayed by an amount equal to twice the propagation time from the transmitterto the reflecting object to obtain an intermediate-frequency signal having the same carrier frequency as that of the output of said second oscillator, and

means including a second detector for converting said intermediate-frequency signal to an audio frequency signal.

WENDELLy L. CARLSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Chaffee Nov. 25, 1941 

